كوكب خارجي

(تم التحويل من Exoplanet)
لمعلومات عن the album by The Contortionist، انظر Exoplanet (album).
Timelapse of exoplanets orbit motion
Four exoplanets of the HR 8799 system imaged by the W. M. Keck Observatory over the course of seven years. Motion is interpolated from annual observations.
Comparison of the size of exoplanets orbiting Kepler-37 to Mercury, Mars and Earth
2 يناير 2013: رواد الفضاء يعلنون أن مجرة درب التبانة تضم أكثر من 400 مليون كوكب خارجي، حيث يستضف كل نجم تقريباً كوكب واحد على الأقل.[1][2][3]
رسم تصوري لكيفية دوران الكواكب حول النجوم في مجرة درب التبانة.[4]

الكوكب خارج المجموعة الشمسية Extrasolar planet، هو أي كوكب يوجد خارج نطاق المجموعة الشمسية (الكواكب التي نعرفعا التابعة للشمس كالأرض والمريخ والمشتري وغيرها). نبحث عن تلك الكواكب في محيط نجوم أخرى من الممكن أن تشكل أنظمة كوكبية فثلما في حالة الشمس. منذ عام 1991 حتى 13 مارس 2012 تم اكتشاف 760 كواكب تقع خارج المجموعة الشمسية. وقد تم اكتشاف وجود معظم تلك الكواكب بطرق غير مباشرة وليس بالرؤية المباشرة لأن قربها من نجم ساطع يجعل رؤيتها مباشرة صعب جداً. In 2016, it was recognized that the first possible evidence of an exoplanet had been noted in 1917, a precovery. حتى 2 يوليو 2026، يوجد 6,316 كوكب خارجي مؤكد في 4,725 نظام كوكبي، 1,055 systems لديه أكثر من كوكب.[5][6]

There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of large planets close to the star.[7][8] About 1 in 5 Sun-like stars[أ] are estimated to have an "Earth-sized"[ب] planet in the habitable zone.[ت][9][10] Assuming there are 200 billion stars in the Milky Way,[ث] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[11]

The least massive exoplanet known is Draugr, which is about twice the mass of the Moon. The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b,[12][13][14] about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium),[15] it is too massive to be a planet and might be a brown dwarf. Known orbital times for exoplanets vary from less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.

The nearest exoplanets are located 4.2 light-years (1.3 parsecs) from Earth and orbit Proxima Centauri, the closest star to the Sun.[16] At the other extreme, there is evidence for extragalactic planets – exoplanets located in other galaxies.[17][18]

The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone (sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[19] In collaboration with ground-based and other space-based observatories, the James Webb Space Telescope (JWST) is expected to give more insight into exoplanet traits, such as their composition, environmental conditions, and habitability.[20]

Rogue planets are those that are not in planetary systems. Such objects are generally considered in a separate category from planets, especially if they are gas giants, often counted as sub-brown dwarfs.[21] The number of rogue planets in the Milky Way is possibly in the billions.[22][23]

التعريف

IAU

The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets.[24][25] The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003.[26] An exoplanet was defined by the following criteria:

  • Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
  • Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
  • Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.[27][28] The official working definition of an exoplanet is now as follows:

  • Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+√621)) are "planets" (no matter how they formed).
  • The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

البدائل

التأكد

An exoplanet is confirmed for NASA's Exoplanet Archive either when "different observation techniques reveal features that can only be explained by a planet"[29] or by analytical techniques.[30] For the Extrasolar Planets Encyclopedia, "A planet is considered as Confirmed if it is claimed unambiguously in an accepted paper or a professional conference."[31]

المسميات

Exoplanet HIP 65426b is the first discovered planet around star HIP 65426.[32]
المقالة الرئيسية: Exoplanet naming convention

The convention for naming exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[33] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.[citation needed]

تاريخ الرصد

NASA graphic of present and future exoplanet missions as of 2022.

For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.[citation needed]

The first evidence of a possible exoplanet, orbiting Van Maanen 2, was recorded in 1917, but was not recognized as such until 2016.[34] The astronomer Walter Sydney Adams produced a spectrum of the star using Mount Wilson's 60-inch telescope which he interpreted the spectrum to be of an F-type main-sequence star. This spectrum was reexamined during studies of white dwarf stars with unpredicted compositions. It is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized by the gravity of the star, the resulting dust then falling onto the star.[35]

الأنواع

تختلف الكواكب الواقعة خارج المجموعة الشمسية عند مقارنتها بكواكب المجموعة:

  • بعضها كواكب غازية أو صخرية ضخمة.
  • بعضها يدور حول أكثر من نجم.
  • بعضها يسبح في الفلك بطلاقة، وبعضها يدور حول أقزام بنية.
  • من الممكن أن يحتوي بعضها على حياة (رغم أن ذلك لم يكتشف بعد).
  • يمكن أن يكونوا أقزام كواكب، وهي كواكب أصغر حجماً وكثافة عن الكواكب العادية.

في فبراير 2009 أعلن عن اكتشاف المسبار كوروت لكوكب يدور حول نجم يبعد عن الأرض بمقدار 400 سنة ضوئية. وأطلق على الكوكب اسم كوروت-إكسو-7ب. ومن الأمثلة أيضا على الكواكب المكتشفة خارج النظام الشمسي كوكب ناري تهطل عليه من نار تحتوي على الصخور والحصى، وذلك حسبما ذكرت وكالة الفضاء الأوروبية. ونقلت صحيفة "تلغراف" البريطانية عن علماء فضاء في جامعة واشنطن الأمريكية قولهم إن الكوكب، الذي أطلق عليه اسم "كوروت -7بي" يعتبر "صورة عن الجحيم".

وأضافوا أنه كوكب صخري يبلغ حجمه ضعفي حجم كوكب الأرض، ويبعد مسافة 1.6 مليون ميل عن شمسه، ما يعني أن هذا الكوكب قريب جداً من شمسه ومحبوس بفعل الجاذبية، مثل القمر مع الأرض. وبسبب قربه من شمسه، فإن درجة حرارة الكوكب عالية جداً، مما يتسبب في ذوبان الصخور على سطحة، فتتصاعد الأبخرة قبل أن تتجمد وتتحول إلى صخور وحصى ثم تهطل مجدداً كوابل من المطر في دورة لا تنتهي. وهنالك أيضا كواكب كثيرة تدور حول نظام شمسي آخر.

خصائص عامة

عدد النجوم والكواكب

أحدث الكواكب الخارجية المكتشفة على بعد 300 سنة ضوئية من النظام الشمسي.


خصائص النجوم المضيفة

التصنيف الطيفي

تصنيف مورگن-كينان الطيفي.



المعدنية

المتغيرات المدارية

Scatterplot showing masses and orbital periods of all extrasolar planets discovered through 2010-10-03, with colors indicating method of detection:
  astrometry
  transit
  timing
For reference, Solar System planets are marked as gray circles. The horizontal axis plots the log of the semi-major axis, while the vertical axis plots the log of the mass.


المحور شبه الرئيسي

الاختلاف المركزي

درجة الميل

الرنين

المدار

توزيع الكتلة

الكثافة والتكوين

مقارنة بين حجم الكواكب بتكوينات مختلفة


الغلاف الجوي

درجة الحرارة

خصائص أخرى

إمكانية الحياة

Artist's impression of Kepler-22b, a "Super-Earth" within its star's habitable zone.


انظر أيضاً

قوائم

تصنيفات

إمكانية الحياة

رواد فضاء

برامج وأجهزة رصد

مهمات

حالية

تحت التطوير

مقترحة

  • ATLAST
  • EChO – for launch in 2024
  • FINESSE
  • New Worlds Mission – for launch in 2019
  • PLATO – for launch in 2024
  • TESS – NASA studied but declined to select for flight. Private funding is now being sought for launch no earlier than 2016[37]

سابقة

مواقع إلكترونية

المصادر

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  3. ^ Cassan, A; Kubas, D.; Beaulieu, J.-P.; Dominik, M.; Horne, K.; Greenhill, J.; Wambsganss, J.; Menzies, J.; Williams, A. (11 January 2012). "One or more bound planets per Milky Way star from microlensing observations". Nature. 481 (7380): 167–169. arXiv:1202.0903. Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108. Retrieved 2012-01-11.
  4. ^ "Planet Population is Plentiful". ESO Press Release. Retrieved 13 January 2012.
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  8. ^ Selection effects – transits, The Open University, An introduction to exoplanets
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  15. ^ خطأ استشهاد: وسم <ref> غير صحيح؛ لا نص تم توفيره للمراجع المسماة bodenheimer2013
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وصلات خارجية

مشروعات بحثية

مراجع

أخبار


خطأ استشهاد: وسوم <ref> موجودة لمجموعة اسمها "lower-alpha"، ولكن لم يتم العثور على وسم <references group="lower-alpha"/>

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